1. Field of the Invention
The present invention relates to an optical intensity modulator and a fabrication method therefor, and more particularly, to an optical intensity modulator having a curved optical waveguide, and its fabrication method.
2. Description of the Related Art
An integrated optical technique means fabrication of several optical devices comprising optical waveguides within one substrate. When using integrated optical techniques, the arrangement of unit optical devices is easy. Thus, a complicated multi-functional device can be easily fabricated in a small area. An optical modulator is one of these integrated optical devices. The optical modulator controls the phase or intensity of an optical wave propagating along an optical waveguide using an external signal. Integrated optical modulators use the electro-optical effect or thermo-optical effect of a medium for forming optical waveguides. Representative electro-optical materials include, for example, a semiconductor (GaAs, InP), a ferroelectric substance (LiNbO.sub.3, LiTaO.sub.3), and a poled polymer. When an electrical field is applied to these materials in one direction, the refractive index of the material in the electrical field direction and in the vertical direction thereto varies. Since a refractive index variation means a change in the phase of optical waves propagated within a medium, phase modulation and intensity modulation can be achieved using this change. In optical modulators using the electro-optical effect, a capacitor-type electrode is formed near an optical waveguide and then a voltage is applied to the capacitor-type electrode to apply an electrical field to the optical waveguide. In contrast with the electro-optical effect, the thermo-optical effect is possessed by almost all optical materials. When the temperature of a material varies, the refractive index of the material is changed due to contraction or expansion of the volume of the material depending on the variation of the material temperature. Accordingly, similar to the electro-optical effect, the thermo-optical effect can be used to obtain phase modulation and intensity modulation of optical waves. In optical modulators using the thermo-optical effect, a micro-heater is fabricated near an optical waveguide, and then a current is applied to the micro-heater to apply heat to the optical waveguide. Since the thermo-optical effect is shown in almost all optical materials, there are various materials to choose from. Also, modulation characteristics can be obtained independent of the polarization of optical waves. However, the thermo-optical effect has a very slow time response property (.about.1 msec) compared with the electro-optical effect. Thus, thermo-optical modulators are generally applied to applications requiring a polarization independent feature rather than being used for high-speed optical signal data transmission.
Integrated optical modulators are roughly divided into intensity modulators using phase modulation, and cutoff optical modulators directly obtaining optical intensity modulation. A Mach-Zehnder interferometric modulator is representative of an intensity modulator using phase modulation. Such an optical modulator is comprised of a substrate, an optical waveguide, and electrodes. The operation of this modulator is described as follows. Light input into the optical waveguide is divided in two, and the two divided light beams are differentially phase-modulated by external electrical fields applied to the electrodes while passing through different paths. If the two optical waves are in-phase at the output end of the optical waveguide, they constructively interfere with each other, so that input optical power is output almost without change. If the two optical waves are out-of-phase, they destructively interfere with each other, so that the optical waves are radiated into the substrate. Thus, output optical power becomes zero.
The cutoff optical modulator is representative of a cutoff optical modulator which directly obtains optical intensity modulation, and is comprised of a substrate, an optical waveguide, and electrodes. The operation thereof is described as follows. When a large voltage is applied to electrodes placed on parts of the optical waveguide, the refractive index of the optical waveguide is changed. When the optical waveguide is cut off by the changed refractive index, guided optical waves are radiated into the substrate, and the output becomes zero.
The interferometric optical intensity modulator using phase modulation requires only phase modulation of light, so that the driving voltage is low, and a well-guiding condition of the optical waveguide can be set. Therefore, the insertion loss of the device is small. However, this interferometric optical modulator complicates the configuration of an optical communications system because of sinusoidal output characteristics with respect to the applied voltage. Also, since the operating point of the optical modulator is sensitive to a change in external factors, for example, temperature, humidity or pressure, many extra devices are required to monitor and compensate for the operating point of the optical modulator. This causes an increase in the cost for constituting an optical transmission system.
The cutoff modulator can solve some of the defects of the above-described interferometric optical modulator. The operating point of the cutoff optical modulator can be set optically, so that a direct current bias for setting the operating point is not required. Accordingly, an operating point drifting phenomenon with respect to the external factors is so small that the cutoff modulator can be used in the optical transmission systems without special extra devices. Also, the cutoff optical modulator exhibits linear output characteristics with respect to the applied voltage, so that it has a wide dynamic range of operation. Thus, the cutoff optical modulator is useful particularly with analog communications systems. In addition, digital output characteristics can be obtained in the guiding conditions of a specific optical waveguide, so that the cutoff optical modulator can be easily applied to digital communications without extra signal processing devices. However, this cutoff optical modulator has a large driving voltage and a high insertion loss. A large change in refractive index is required to cut off waveguiding, and an extinction ratio of about 20 dB can be obtained by generally applying a voltage of tens of volts or greater. Furthermore, the initial waveguiding conditions of an optical waveguide must be set near a cutoff area according to an operational principle, thus the insertion loss is large.